Moisture content in wood is an important factor in a number of properties of structural engineered wood products (such as oriented strand board (“OSB”), plywood, scrimber, and the like). One property of particular interest to purchasers and users of such wood products is the modulus of elasticity (“MOE”). The MOE is a measure of the stiffness of the wood board. For short distance traversal, such as for a window, the MOE need not be as high as for a wood beam spanning, for example, a two-car garage, which beam requires greater strength and stiffness to prevent sagging over the longer distance and greater weight stress. High MOE is less important for plywood and oriented strand board than for other engineered products. “Chapter 4: Mechanical Properties of Wood,” by David W. Green et al., in “Wood Handbook: Wood as an Engineering Material” (Madison, Wis. USDA Forest Service, Forest Products Laboratory, 1999, General Technical Report FPL; GTR-113: pages 4.1-4.45), provides a general discussion of wood properties.
Old growth unprocessed wood generally has a higher MOE than in new growth unprocessed wood or pulpwood. Pulpwood is commonly defined as wood that is 12-60 years of age or of a certain diameter (to be distinguished from veneer or dimension lumber). Old growth trees are rapidly vanishing as forests are depleted. New “immature” tree farms are increasing in development to provide a nearly limitless source of such wood. Such farms can grow trees at a faster rate using modern technology. However, the MOE of the immature trees is often less than the MOE of old growth trees. High MOE is desirable for use in some engineered wood products as it can withstand a higher load (i.e., is stronger), hence, the immature wood timber must be processed. Producing a product which has a high and desirable MOE has become increasingly difficult and more expensive with new growth trees. Current manufacturing processes using pulpwood are not effectively or practicably making product with an MOE of 1.5 or higher. It would be desirable to have an engineered wood product made of pulpwood with a MOE of at least 1.8.
Further, the modulus of rupture (MOR) is also an important characteristic of engineered wood products. A higher MOR is indicative of a product that is stronger.
Processing the trees into engineered products involves a number of steps. Among the steps is forming strand-like mats of crushed fibers and drying them to a target moisture content in a first drying step. Another step is then adding resin in an aqueous solution to separate strands of scrim to bind them together. After this step the billet of material is dried to a target moisture content level, typically by heating or microwaving the wood, usually under pressure. The water content in the resin is a factor in the drying step to drive off residual water. If the moisture level is driven too low in the first drying step, the wood will not adequately accept the resin and will not form a structurally sound lumber product.
Presently, an MOE of about 2.0 is considered to be ideal for applications requiring substantial stiffness and strength. Because of the shortage of old growth timber, the MOE which customers have had to accept has degraded to 1.8-1.9 in certain situations.
It would be desirable to have a process for making engineered wood products in which the MOE could be controlled during the production process. It would also be desirable to have such a process which could use pulpwood (inherently having lower MOE than old growth trees) and produce an engineered product having a high desirable MOE.